Method and apparatus for screening pulp



May 15, 1962 H. F. SZEPAN METHOD AND APPARATUS FOR SCREENING PULP 3 Sheets-Sheet 1 Filed July 2, 1959 May 15, 1962 H. F. SZEPAN 3,034,650

METHOD AND APPARATUS FOR SCREENING PULP Filed July 2, 1959 5 Sheets-Sheet 2 INVENTOR.

H ENRV F. SZEPAN BY ATTORNEY May 15, 1962 H. F. SZEPAN METHOD AND APPARATUS FOR SCREENING PULP 3 Sheets-Sheet 3 Filed July 2, 1959 INVENTOR. HENRY F. SZEPAN ATTORNEY United States Patent METHOD AND APPARATUS FOR SCREENING PULP Henry F. Szepan, 295 Amherst St., Manchester, N.H. Filed July 2, 1959, Ser. No. 824,732 4 Claims. (Cl. 209273) The present invention involves a pulp screen of high capacity and extreme versatility, the latter feature involving extreme ease of adjustment in the field so as to do an optimum job on the particular pulp for which the screen may be selected.

Screens of this type are typified in the patent to Bowen, No. 2,845,848 dated May 8, 1958 and in the application of Rich and Luthi Serial No. 711,704 filed January 28, 1958 now United States Patent No. 2,908,390, issued October 13, 1959. The present invention is an improvement on both of the foregoing. In each of the foregoing, attention is paid to operating on pulp at maximum consistency, thereby increasing the tons per day capacity of a given machine, and also performing, in effect a tWo stage screening of the pulp in a single machine with minimum pumping costs for dilution water and for feeding the machine. These inventions also were concerned with ready regulation of the richness of the rejects, which, if perfect, would be zero, that is, there would be no good fibre entrapped with the rejects. The present invention just about does this and represents an improvement in degree over the two cited above.

The present invention differs from the foregoing in that it is adaptable throughout the pulp screening range from a knotter, handling the coarsest pulp to the finest classification of acceptable fibre. Emphasis cannot be too great on the fact that the behaviour of pulp screens is the least predictable of all operations in the paper making process, which still remains after centuries, more an art than a science.

Reference is hereby made to Pulp and Paper Manufacture, vol. 1 (McGraw-Hill, 1950) chapter 6, pages 694- 756 in which the fundamentals of the pulp screen problem are well covered. The cited chapter makes abundantly clear that the problems are many and varied, and that for any piece of equipment in this field, versatility is a most desirable factor.

An object of this invention is to provide a pulp screen which, in its fundamental design will accomplish at a single pass, both primary and secondary screening.

It is a further object of this invention to provide a pulp screen which is easily adjustable in the field to d optimum screening on any given pulp.

It is a further object of this invention to provide a pulp screen operable at maximum consistency and minimum horsepower per daily ton of pulp screened.

It is a further object of this invention to provide a pulp screen which will reject a minimum of truly acceptable stock.

The above and other objects will be made clear from the following detailed description taken in connection with the annexed drawings in which:

FIGURE 1 is a front elevation vertically sectioned;

FIGURE 2 is a plan view of the rotor shown in FIG- URE 1; and

FIGURE 3 is a section on the line 3-3 of FIGURE 2.

In the patent to Bowen there is disclosed a fixed screen and a rotor with a pulp inlet at one end and a rejects outlet at the other, a central diaphragm or baffle with dilution on the side of the baffle adjacent the rejects outlet.

The Rich-Luthi application improved on the Bowen patent by providing a volute entryway for the pulp, plus a pitched blade stator to direct the incoming pulp in the same rotational direction as that of the rotor. In both machines the rot-or blades are pitched to feed the pulp 3,li34,650 Patented May 15, 1962 7 toward the rejects end and are sloped relative to the radii to provide in combination, increased centrifugal force on the stock plus a cavitation efiect which serves to clean the screen. In each case, also, the central or median baffle ofiers a dividing line or plane between primary and secondary screening.

The present invention improves upon the foregoing by reversing the pitch of the rotor blades on the trailing side of the baffle and by moving the baffle from its heretofore median position to a plane nearer the rejects outlet. This provides a longer period of primary screening with increased retention period in the secondary phase. The result is to permit greater inlet consistency and greater rejects consistency, the latter being due to the longer retention in the secondary phase so that less dilution water is necessary. In consequence, capacity is increased with minimum volume of over-run.

Referring now to FIGURE 1, there is shown a screen casing 10 in which is mounted a fixed screen 12. The casing 10 has a volute inlet 14 as disclosed in the Rich- Luthi application aforesaid. Adjacent the volute inlet 14- is a stator 16 for converting the inlet velocity head to a swirling action. A shaft 18 penetrates the casing 10 through stuffing boxes 20 and bearings 22. Acceptable stock passes through the screen 12 and emerges through an outlet in casting 24. Rejected stock emerges through an outlet 26. Dilution water is supplied through a shower pipe 27.

A diaphragm 2.8 is keyed to the shaft 18 and carries rotor blades 34 which receive additional support from rings 32. On at least two diametrically opposed rotor blades 30 are afiixed paddles or agitators 34 which serve to clear the entrance of the rejects outlet 26.

Referring now to FIGURES 2 and 3 it will be seen that diaphragm 28 and the rings 32 are each made in two parts which are bolted together, diaphragm 28 as shown at 36 and the rings 32 as shown at 38.

The direction of rotation is as shown by arrow Y in FIGURE 3. Blades 30 are backswept a few degrees from the direction of rotation as best shown in FIGURE 3 and from the inlet end to the diaphragm 28 have a mild helicoidal pitch tending to feed the stock from the inlet toward the diaphragm 28. On the trailing side of the diaphragm 28, however, each blade 30 has a r versally helicoidally pitched portion 39'. It will be noted that the diaphragm 28, instead of being at the median point of the rotor is approximately three fourths of the Way from the inlet to the rejects outlet. It is further to be noted that the reverse pitch portions 30 are cantilevers not supported by a ring 32. This is important, since it permits adjustment of the reverse pitch in the field so as to provide optimum performance for any given stock. To illustrate this importance, a rotor was tested with zero retarding angle on the portions 30'. With this setting total rejects were found to contain as much as 50 percent good fibre. Portions 30 were then bent to a five degree negative angle after which the rejects were found to contain only ten to fifteen percent good fibre. Thereafter the portions 30' were bent further to a negative angle of twenty degrees, whereupon the good fibre in the rejects dropped to less than five percent so that the rejects could be considered fibre-free for all practical purposes.

The number of blades and the speed of rotation depend on the nature of the stock and the size and number of the perforations in the screen 12. When the screen is used as a knotter, the perforations are between 716 inch and /2 inch in diameter. Four blades With a five degree pitch on the inlet side, a twenty degree negative pitch of the portions 30 and a rotational speed of 225 to 250 rpm. gives excellent results both in total capacity and in a small percent of good fibre in the rejects. On the other hand, for difi'icult screening such as groundwood or pre-refined pulp, ten blades with a negative pitch of 30 degrees of the portions 30 was found to give optimum performance at 350 rpm.

It is clear from the foregoing that there Will be neces sarily considerable variation in constructional detail depending on the desired capacity and the nature of the pulp to be screened. This invention is not, therefore, to be limited to the precise details disclosed herein but only as set forth in the subjoined claims.

I claim:

1. A method of screening pulp comprising: advancing a pulp slurry along a helicoidal path within a cylindrical screen; constricting the path at a predetermined plane; reopening the path and simultaneously diluting the slurry, and, during such dilution, retarding the aXial progress of the slurry.

2. A method as set forth in claim 1 in which the plane of constriction is substantially nearer the trailing than the leading-end of the path.

3. Apparatus for screening pulp comprising: a horizontal, cylindrical screen; a rotor inside said screen; a diaphragm on said rotor; blades attached to said diaphragm, said blades heing pitched in opposite directions on opposite sides of said diaphragm.

4. Apparatus as set forth in claim 3 in which the diaphragm is substantially nearer one end than the other of said rotor.

References Cited in the file of this patent UNITED STATES PATENTS 2,380,321 Lykken July 10, 1945 2,752,828 Sandison July 3, 1956 2,835,173 Martindale May 20, 1958 2,845,848 Bowen Aug. 5, 1958 2,908,390 Rich et al. Oct. 13, 1959 

